The present state of the art with respect to soldering is such that in production line formation of soldered joints, as well as in the formation of soldered joints to build apparatus from a kit, it has been found that approximately 90% of all rejections in quality control are the result of the inefficacy of the solder joints which have been formed.
In high performance devices which are intended to operate at all times and provide a certainty and continuity of operation which must be depended upon by appropriate personnel (such as in military applications as for instance rocket silos) the quality control and operational control check systems provide for daily review of the continuity of connections of all soldered joints in the entire system because of the comparatively high probability that solder joint degeneration will occur owing to inefficacy of the original solder joint and the continuous thermal expansion and contraction which is imposed on the solder joint.
In present soldering methods, the splice between two or more metal parts which is to be electrically soldered, by the application of a lower metal melting temperature metal alloy for soldering the soldering is initially performed with a rise in flux in order to obtain a flash cleaning of the spliced surfaces and of the soldering iron itself.
The soldering iron is usually a copper alloy that conducts heat developed for melting the solder onto the solder itself. Depending on the soldering apparatus, whether it is a straight soldering iron with a remote heating element or a soldering gun style with a copper alloy tip that provides in itself a resistance heating element, the control of the melt point of the solder and of the length of heat application to the solder to cause the melt results in extraordinarily wide temperature ranges. The splice can therefore be left with too low a peak temperature to complete a low resistance splice, or so overheat the splice as to damage associated heat sensitive components. The present state of the art even with installation of controls of various types with respect to temperature levels and timing does not produce a predictable splice nor a repeatable splice nor does it provide any warning of heat damage or of further problems which may arise from too cold or too hot a splice.